Radio frequency (“RF”) communications can be defined by a “link budget”. A link budget includes the addition and subtraction of gains and losses along an RF communication link. When the gains and losses of various components along the RF link are determined and summed, the system performance can be estimated. At a receiver, the signal-to-noise ratio (“SNR”) must be above a certain threshold, for a given bit error rate (“BER”) probability, in order for successful communication to take place. It is an objective of system designers to improve the link budget while at the same time lowering the overall cost of the system. With respect to either the uplink direction, that is the direction from a mobile to the network satellite or base transceiver system (BTS), or the downlink direction from the network satellite or BTS to the mobile, if the link budget is improved, less power will be required, and a smaller mobile unit with a longer battery life can be provided. Further, the mobile unit will not need to have as large an antenna.
Factors included in the link budget include the power amplifier output power, transmit antenna gain, slant angles and corresponding atmospheric loss over distance, transponder noise levels and power gains, receive antenna and amplifier gains and noise factors, cable losses, interference levels, and attenuation factors due to climate conditions. It is the goal of system designers to obtain a link budget improvement in various areas of the communication path. One technique to improve the link budget is to use a technique known as “diversity”.
Diversity allows for the use of multiple communication paths between transmitters and receivers. This diverse path may, under certain conditions, turn out to be the optimal signal path, as opposed to the “primary” path. By enabling communication among different paths, an optimal and efficient communication system can be developed.
The use of satellites for personal and business communications is an important technique in global communication networks. While satellite communication systems provide obvious benefits that allow users to transmit and receive communication signals over a large footprint, these systems are not without their inherent problems. Diverse satellite systems allow system designers to combine the signals from multiple satellites. One known method is the maximum ratio combining technique where complex amplitudes are weighted according to the received signal strength. In other words, if two signals are received, each from a different satellite, the stronger of the two signals is given greater weight, since it is less error prone.
The use of multiple satellites in a diversity system can result in a link budget improvement. However, a problem associated with satellite diversity is that the satellites are situated at different locations and the propagation time is different between a mobile or terrestrial station and the satellites. Another problem associated with satellite communication systems is inter-symbol interference (ISI). ISI arises when there is a distortion of the received communication signal. This distortion results from the overlap of individual pulses to the degree that a receiver cannot distinguish between individual signal elements. Methods of solving the problem of ISI can often involve complicated equalizing schemes for non-OFDM based systems. A much simpler and effective way to address the ISI problem in OFDM systems is via the use of cyclic prefixes.
A modulation scheme that is used in satellite and some forms of terrestrial communications is Orthogonal Frequency Division Multiplexing (OFDM). The OFDM modulation technique transmits large amounts of data over a radio wave by splitting the signal into multiple smaller sub-signals that are then transmitted simultaneously to the receiver at closely spaced frequencies or sub-carriers. The OFDM modulation technique allows for the use of a cyclic prefix to address the problem of inter-symbol interference (ISI). An OFDM symbol can be extended by the use of a cyclic prefix (CP). In one common technique to generate the CP at the transmitter, the last part of each OFDM symbol is inserted at the start of the same symbol. At the receiver, the portion of the signal within the cyclic prefix of the OFDM symbol mitigates the distortion caused by the multi-path propagation of the radio link. If multiple signals are received due to multi-path propagation, ISI is prevented if the relative delay is less than the CP. Any delay greater than the cyclic prefix length causes ISI, while any delay less than the cyclic prefix length avoids ISI.
In the uplink portion from mobile station to satellite of the communications system, each mobile station may be allocated a subset of different OFDM sub-carriers. Because each mobile station is situated in a different location and has a different propagation delay to the satellite, they each must be synchronized in such a way so that the signals all arrive at the satellite or base transceiver station (“BTS”) at the same time, or at least within the CP, in order to avoid ISI. If the transmission is received outside of the CP, there will be excessive interference and the communication performance will be reduced. Thus, for example, for a communications system having one or more satellites in or near geostationary orbit around the Earth, a transmission from a transmitter in nearer the equator, such as from a mobile station located in the State of Texas, arrives at the satellite sooner than a transmission located in a more northern latitude, such as from Canada, because Texas is physically closer to the satellite. The process of establishing timing and power alignment through a communications link is sometimes called “ranging”.
A ranging problem that often occurs with a diverse satellite system is that due to the fact that the satellites are at different locations, the propagation time between the mobile stations and the satellites is different. If some mobiles “range” through one satellite and other mobiles “range” through other satellites, the diverse signals may be more than the CP apart in time, resulting in a timing misalignment and causing ISI to occur. Further, the link budget may not be improved by the requisite amount.
What is therefore needed is a method and system that resolves the timing misalignment issue that occurs during the ranging and transmission processes in a diverse communication system while improving the overall link budget.